I think there are rules, and then there are always, sooner or later, these "grey area" cases, and in the end, it's up to the owner to make a judgement call.
If he feels like C-arf is getting too much for nothing, then he's free to zap that post.
I think all of us can actually live quite happily with straight answers, like "I zapped that post slamming that product because the maker is a sponsor and we need the money to have any forum at all" or "I zapped that post because they refused to advertise, I don't want to give them free advertising anyway" or whatever.
I think people will find it refreshing and honest IF such is the case, I think people get most upset when they feel something is happening behind the scenes that they do not know about. SOME will always cry foul, no matter what, no point in trying to please them.
But me, at least, I can live happily within certain commercial restraints imposed by the need for sponsors, so, in this case, Marc has already said enough to satisfy ME, he does not REALLY owe any explanation either way, but I thought his answer was pretty straight-up.